Hepatocyte growth factor (HGF) is a growth factor that can be found in human serum. The HGF receptor (HGFR) has been identified as the product of the c-Met proto-oncogene. See, e.g., Bottaro et al., Science, 251:802-804 (1991); Naldini et al., Oncogene, 6:501-504 (1991); WO 92/13097; and WO 93/15754.